Skew-axis gearing shows its teeth

Spiradrive is the name given to a specific type of skew-axis gearing - gearing in which the axis of the gear and pinion are at right-angles but do not intersect. Spiradrive gears are sometimes referred to as screw-type gearing, this being a general description of the meshing action of the gear and pinion (the equivalent description for spur and bevel gears would be rolling-type gears). Spiradrive gearsets comprise two members, namely a gear and a pinion.

By definition the pinion has fewer teeth (or threads), and is normally the driving member, and the driven member is a face type gear resembling a spiral bevel or hypoid gear.

Spiradrive gears are unusual in that, unlike other forms of gearing such as spur, helical and bevel gears, their design does not involve the use of standard involute tooth shapes and conventional tooth pitch sizes such as diametral pitch (DP) and metric modular pitch (Mod).

A Spiradrive gearset is designed to suit the requirements of the application - the size and shape of the teeth being determined by those requirements.

This unique characteristic enables the designer to exploit Spiradrive in many different ways.

The ability to obtain far higher ratios in a single Spiradrive gear and pinion set, than is possible with any other form of gearing, means that the number of components in the drive can be reduced and hence its overall size and manufacturing cost.

The use of freely available conventional steels, with their high torque transmission capabilities, in place of the less strong and more expensive alloy bronzes, enables the designer to reduce the material cost of the gear package.

The geometry of a Spiradrive gearset is such that, apart from applications where minimal to zero backlash is required, the mounting distance does not have to be controlled to the fine limits required by other gear forms, thus helping to reduce the machining time for the housing and facilitating easier assembly.

Furthermore, the face form of the gear lends itself to high production techniques such as die-casting, sintering, plastic moulding and precision forging, and the pinion can be thread rolled if required in economical batch quantities.

Understanding that Spiradrive gears do not involve the use of standard tooth and pitch sizes, the designer is then able to design into them specific performance characteristics, such as high efficiency, self-locking, reversibility, high strength, exceptionally smooth angular velocity etc.

The permutations of these features are almost unlimited.

A Spiradrive gear generally has 10-12% of its teeth in simultaneous contact with the mating pinion, as opposed to a worm gear which, at the most, has only three teeth in simultaneous contact with its mating worm.

This means that the designer can make the teeth of the Spiradrive gear much smaller than those of the worm gear, as the load being transmitted can be spread over a greater number of teeth.

In turn, this means a reduction in the overall size of the gear envelope.

The generally accepted upper ratio limit for a worm gearset is 90:1.

Above this figure the outside diameter becomes extremely large relative to the outside diameter of the worm, and the designer has to resort to the use of multistage reductions to achieve the required ratio.

This problem does not occur with Spiradrive gears where ratios from 5:1 to 400:1 can be accommodated in a single, well-proportioned gearset Spiradrive pinions have either a tapered or a parallel outside diameter, the choice being dependent on the requirement of the application.

The cylindrical or zero taper angle (ZTA) type can be used for ratios down to 5:1, and is more likely to be used where maximum rigidity is required.

Both types can be used for ratios from 10:1 upwards.

Accurate control of backlash is yet another useful feature of Spiradrive gearing.

Using a tapered pinion, it is possible to attain a zero backlash condition - coarse adjustment is made by moving the gear axially into or out of contact with the pinion, and fine adjustment by moving the pinion into or out of contact with the gear.

In practice the degree of backlash obtained is a function of the accuracy built into the gear and pinion at the manufacturing stage.

When designing a Spiradrive gearset it is essential to know the application for which it is intended, the torque and strength requirements, the quantities required, and the probable lubrication arrangements, as there are many material combinations that can be used.

The most commonly used material for a pinion is hardened steel, but soft steel or brass can be used for lightly loaded applications.

Gear materials range from bronze, which has proved to be the most suitable material for applications involving moderate loads or precision drives, to hardened steel, which is invariably used for heavy duty power transmission applications.

Alternatively, gears required in volume for use on light load, or intermittent running applications can be produced in sintered iron, die-cast aluminium or nylon as those materials are particularly suited to modern high volume production techniques.

The geometry of a Spiradrive gearset is such that a hydrodynamic film tends to build up ahead of the point of contact of the pinion with the gear teeth.

This ensures that the gear teeth always come into contact with freshly lubricated surfaces on the pinion and contributes to the overall the efficiency of the drive.

The use of the correct lubricant is an important consideration when designing a Spiradrive gear drive.

The ideal lubricant for Spiradrive gearsets where both the pinion and the gear are made in hardened steel is extreme pressure (EP) type oil, as used in automotive hypoid drives.

True EP lubricants contain additives such as sulphides, chlorides and phosphates, which combine with the base material of the gearset to form anti-weld compounds.

These compounds, and in particular the sulphur ones, have a low shear strength and a relatively low melting point which prevent welding of the tiny asperities on the contacting tooth surface and thus eliminate metal transference, which on a large scale would obviously result in serious damage.

EP oils can also be used on gearsets comprising a steel pinion and a bronze gear, but a conventional heavy duty worm wheel oil will serve equally as well.

Various greases are available for use on lightly loaded applications and applications where the dissipation of generated heat are not a problem.

Choosing the correct materials and lubricants is a complex subject far too lengthy to discuss here: suffice it to say that a wide choice of materials and lubricants is available providing great design flexibility, economic production and extended operative life.

Davall Gear Company can advise on all aspects of the design of Spiradrive gearsets to suit any given application.